Method and apparatus for calibrating print head pressure and applying an accurate print pressure during production

ABSTRACT

A stencil printer includes a frame, a stencil coupled to the frame, a substrate support coupled to the frame to support a substrate in a print position, and a print head, coupled to the frame, to deposit and print viscous material over the stencil. The print head includes a squeegee assembly having at least one squeegee blade and a squeegee blade movement mechanism configured to move the squeegee blade from a raised position in which the squeegee blade is spaced from stencil and a lowered position in which the squeegee blade engages and applies a force on the stencil. The print head further has a device to detect a first reference point associated with a first force of the squeegee blade against the stencil and a second reference point associated with a second force of the squeegee blade against the stencil when moving the squeegee blade to the lowered position in which the squeegee blade engages and applies a force on the stencil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/786,971, filed Apr. 13, 2007, entitled METHOD AND APPARATUS FORCALIBRATING PRINT HEAD PRESSURE AND APPLYING AN ACCURATE PRINT PRESSUREDURING PRODUCTION, which is currently pending and incorporated herein byreference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates generally to methods and apparatus for printingviscous material, such as solder paste, onto a substrate, such as aprinted circuit board, and more particularly to a method and apparatusfor calibrating the pressure or force of squeegee blades of a print headon a stencil, and for applying an accurate print pressure duringproduction.

2. Discussion of Related Art

In a typical surface-mount circuit board manufacturing operation, astencil printer is used to print solder paste onto a printed circuitboard. A circuit board, broadly referred to as an electronic substrate,having a pattern of pads or some other conductive surface onto whichsolder paste will be deposited, is automatically fed into the stencilprinter. One or more small holes or marks on the circuit board, calledfiducials, is used to align the circuit board with the stencil or screenof the stencil printer prior to the printing of solder paste onto thecircuit board. The fiducials serve as reference points when aligning acircuit board to the stencil. Once a circuit board has been aligned withthe stencil in the printer, the circuit board is raised to the stencilby a substrate support, e.g., a table having pins or other work holders,and fixed with respect to the stencil. Solder paste is then dispensed bymoving a wiper blade or squeegee across the stencil to force the solderpaste through apertures formed in the stencil and onto the board. As thesqueegee is moved across the stencil, the solder paste tends to roll infront of the blade, which desirably causes mixing and shearing of thesolder paste so as to attain a desired viscosity to facilitate fillingof the apertures in the screen or stencil. The solder paste is typicallydispensed onto the stencil from a standard solder paste supplycartridge. After the print operation, the board is then released,lowered away from the stencil, and transported to another station withinthe printed circuit board fabrication line.

During a print cycle, as described above, the squeegee is moved acrossthe stencil to force solder paste or any other viscous material throughapertures formed in the stencil. FIG. 1 schematically illustrates theconnection of a squeegee blade to a print head. As shown, a squeegeeblade 40 is secured to a squeegee blade holder 64 in a position in whichthe squeegee blade may be disposed at an angle with respect to a stencil18. The print head includes a first movable plate 54 and a secondmovable plate 56 that is connected to the first movable plate. The firstmovable plate 54 is secured to a frame (not shown) of the print head bytwo linear bearings indicated at 57 and driven by a lead screw 58, whichis driven by a motor (not shown) provided in the print head. Thearrangement is such that the lead screw threadably engages a lead nut 60secured to the first movable plate to move the first and second movableplates 54, 56 along a path defined by the linear bearings 57. The secondmovable plate 56 includes a blade holder 64 to secure the squeegee blade40 to the second movable plate. As shown, a compression spring 62 isdisposed around the lead screw 58 to provide a resistance force betweenthe first movable plate 54 and the second movable plate 56.

One issue with printing is calibrating the force upon which the squeegeeblade 40 engages the stencil 18. Specifically, the spring constant ofthe compression spring 62 may vary. Also, when using a flexible(non-rigid) squeegee blade, the spring constant of the squeegee blademay effect the force upon which the blade engages the stencil 18. Withreference to FIG. 1, a sensor 52 may be provided to a home positionand/or a distance of the first movable plate 54 with respect to theframe of the print head. With the known calibration methods, e.g.,replacing the squeegee blade 40 with a calibration gauge, the force ofthe blade against the stencil is determined by moving the first movableplate 54 a known distance, which is dependent upon the spring constantof the compression spring 62. Thus, if a customer has a completely rigidsqueegee blade, the force of the blade against the stencil is somewhataccurate. It is difficult to determine the force of the squeegee bladeagainst the stencil when the blade is flexible or when the springconstant of the compression spring is not to tolerance.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide improvements to stencil supportassemblies, such as those described above.

A first aspect of the invention is directed to a stencil printer forprinting viscous material on a substrate. The stencil printer comprisesa frame, a stencil coupled to the frame, a substrate support coupled tothe frame to support a substrate in a print position, and a print head,coupled to the frame, to deposit and print viscous material over thestencil. The print head comprises a squeegee assembly comprising atleast one squeegee blade and a squeegee blade movement mechanismconfigured to move the at least one squeegee blade from a raisedposition in which the at least one squeegee blade is spaced from stenciland a lowered position in which the at least one squeegee blade engagesand applies a force on the stencil. The print head further comprises adevice to detect a first reference point associated with a first forceof the at least one squeegee blade against the stencil and a secondreference point associated with a second force of the at least onesqueegee blade against the stencil when moving the at least one squeegeeblade to the lowered position.

Embodiments of the stencil printer may further be directed to having theat least one squeegee blade movement mechanism comprising a firstmovable member coupled to the frame of the stencil printer and a secondmovable member coupled to the first movable member and to the at leastone squeegee blade. The squeegee blade movement mechanism furthercomprises a lead screw housed by the frame of the stencil printer and alead nut secured to the first movable member and threadably engaged withthe lead screw to move the first and second movable members so as tomove the at least one squeegee blade between the raised and loweredpositions. The squeegee blade movement mechanism further comprises acompression spring disposed around the lead screw to provide aresistance force between the first movable member and the second movablemember. The arrangement is such that when moving the at least onesqueegee blade to the lowered position against the stencil so as toapply a force on the at least one squeegee blade, the second movablemember moves toward the first movable member against the resistance ofthe compression spring. The second movable member includes a squeegeeblade holder to secure the at least one squeegee blade to the secondmovable member. The device comprises a flag secured to one of the firstmovable member and the second movable member and a sensor secured to theother of the first movable member and the second movable member. Thesensor is configured to detect at least two features of the flag whenmoving the at least one squeegee blade to a position of an applied forcefrom the lowered position. The first and second reference points areassociated with the at least two features of the flag. A gauge, whichreplaces the at least one squeegee blade, is provided to measure a forceof the gauge against the stencil. The gauge is configured to measure asimulated force of the at least one squeegee blade against the stencilwhen the first reference point of the flag is detected by the sensor andwhen the second reference point of the flag is detected by the sensor.

Another aspect of the invention is directed to a stencil printer forprinting viscous material on a substrate. The stencil printer comprisesa frame, a stencil coupled to the frame, a substrate support coupled tothe frame to support a substrate in a print position, and a print head,coupled to the frame, to deposit and print viscous material over thestencil. The print head comprise at least one squeegee blade, a firstmovable member coupled to the frame of the stencil printer, a secondmovable member coupled to the first movable member and to the at leastone squeegee blade, the first and second movable members beingconfigured to move the at least one squeegee blade from a raisedposition in which the at least one squeegee blade is spaced from stenciland a lowered position in which the at least one squeegee blade engagesand applies a force on the stencil, a flag secured to one of the firstmovable member and the second movable member, and a sensor secured tothe other of the first movable member and the second movable member. Thesensor is configured to detect the flag when moving the at least onesqueegee blade. The flag and sensor is configured to detect a firstreference point associated with a first force of the at least onesqueegee blade against the stencil and a second reference pointassociated with a second force of the at least one squeegee bladeagainst the stencil when moving the at least one squeegee blade to thelowered position.

A further aspect of the invention is directed to a method of calibratinga squeegee blade force against a surface. The method comprises: moving agauge in a downward direction against the surface; continuing moving thegauge in a downward direction to a first reference point; measuring theforce of the gauge against the surface; continuing moving the gauge in adownward direction to a second reference point; and measuring the forceof the gauge against the surface.

Yet another aspect of the invention is directed to a method ofcalibrating a squeegee blade force against a stencil in a stencilprinter. The method comprises: (a) removing a stencil from a stencilprinter; (b) removing squeegee blades from the stencil printer; (c)attaching a gauge to a bottom of a print head; (d) moving the gauge todetect a first position and a second position; and (e) recording a forceparameter associated with first position and a force parameterassociated with the second position. An embodiment of the method furthercomprises: (f) loading the stencil into the stencil printer; (g)removing the gauge; (h) re-installing the squeegee blades to the stencilprinter; (i) moving at least one squeegee blade to contact the stencil;(j) moving the at least one squeegee blade to the first position and tothe second position; (k) recording a force parameter associated with thefirst position and a force parameter associated with the secondposition; and (l) calculating and applying a desired print force basedupon an interpretation of the calibrated positions of first and secondforce values and actual positions of the first and second force valueswith the at least one squeegee blade in place.

Another aspect of the invention is directed to a print head to depositand print viscous material over the stencil. The print head comprises aframe, at least one squeegee blade, a first movable member coupled tothe frame, a second movable member coupled to the first movable memberand to the at least one squeegee blade, the first and second movablemembers being configured to move the at least one squeegee blade from araised position in which the at least one squeegee blade is spaced fromstencil and a lowered position in which the at least one squeegee bladeengages and applies a force on the stencil, a flag secured to one of thefirst movable member and the second movable member, and a sensor securedto the other of the first movable member and the second movable member.The sensor is configured to detect the flag when moving the at least onesqueegee blade. The flag and sensor are configured to detect a firstreference point associated with a first force of the at least onesqueegee blade against the stencil and a second reference pointassociated with a second force of the at least one squeegee bladeagainst the stencil when moving the at least one squeegee blade to thelowered position.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic view of a prior art print head assembly;

FIG. 2 is a front perspective view showing a stencil printer of anembodiment of the invention;

FIG. 3 is a perspective view of the stencil printer shown in FIG. 2showing a print head assembly of an embodiment of the invention;

FIG. 4 is a perspective view of the print head assembly shown in FIG. 2with squeegee blades being removed to more clearly show movable membersof the print head assembly;

FIG. 5 is an enlarged perspective view of the print head assembly withselected parts removed;

FIG. 6 is an even further enlarged perspective view of a sensor flag ofthe print head assembly; and

FIG. 7 is a schematic view of a squeegee blade (front or rear) of theprint head assembly of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein, is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

For purposes of illustration, embodiments of the invention will now bedescribed with reference to a stencil printer used to print solder pasteonto a printed circuit board. One skilled in the art will appreciate,however, that embodiments of the invention are not limited to stencilprinters for printing solder paste, but may also include printing othermaterials, such as adhesives, epoxies, underfill materials andencapsulant materials. Also, the terms screen and stencil may be usedinterchangeably herein to describe a device in a printer that defines apattern to be printed onto a substrate.

Referring now to the drawings, and more particularly to FIG. 2, there isgenerally indicated at 10 a stencil printer of an embodiment of theinvention. As shown, the stencil printer 10 includes a frame 12 thatsupports components of the stencil printer. The components, in part, mayinclude a controller 14, a display 16, a stencil 18, and a print headassembly or print head generally indicated at 20 configured to apply thesolder paste. Each of these components may be suitably coupled to theframe 12. In one embodiment, the print head 20 is embedded within on aprint head gantry 22, which enables the print head to be moved in they-axis direction under the control of the controller 14. As describedbelow in further detail, the print head 20 may be placed over thestencil 18 and a front or rear squeegee blade of the print head may belowered in the z-axis direction into contact with the stencil. Thegantry then may be moved across the stencil to allow printing of solderpaste onto a circuit board.

Stencil printer 10 may also include a conveyor system having rails 24,26 for transporting a circuit board to a print position in the stencilprinter. The stencil printer 10 has a support assembly 28 to support theprinted circuit board (or “substrate”), which, as will be described ingreater detail below, raises and secures the printed circuit board sothat it is stable during a print operation. In certain embodiments, thesubstrate support assembly 28 may further include a particular substratesupport system, e.g., a solid support, a plurality of pins or flexibletooling, positioned beneath the circuit board when the circuit board isin the print position. The substrate support system may be used, inpart, to support the interior regions of the circuit board to preventflexing or warping of the circuit board during the print operation.

In one embodiment, the print head 20 may be configured to receive solderfrom a source, such as a dispenser, e.g., a solder paste cartridge, thatprovides solder paste to the print head during the print operation.Other methods of supplying solder paste may be employed in place of thecartridge. In one embodiment, the controller 14 may be configured to usea personal computer having a Microsoft DOS or Windows XP operatingsystem with application specific software to control the operation ofthe stencil printer 10. The controller 14 may be networked with a mastercontroller that is part of a line for fabricating circuit boards.

In one configuration, the stencil printer 10 operates as follows. Acircuit board is loaded into the stencil printer 10 using the conveyorrails 24, 26. The support assembly 28 raises and secures the circuitboard to a print position. The print head 20 then lowers the desiredsqueegee blade of the print head in the z-axis direction until squeegeeblade of the print head contacts the stencil 18. The print head 20 isthen moved in the y-axis direction across the stencil 18. The print head20 deposits solder paste through apertures in the stencil 18 and ontothe circuit board. Once the print head has fully traversed the stencil18, the squeegee blade is lifted off the stencil and the circuit boardis lowered back onto the conveyor rails 24, 26. The circuit board isreleased and transported from the stencil printer 10 so that a secondcircuit board may be loaded into the stencil printer. To print on thesecond circuit board, the other squeegee blade is lowered in the z-axisdirection into contact with the stencil and the print head 20 is movedacross the stencil 18 in the direction opposite to that used for thefirst circuit board.

Still referring to FIG. 1, an imaging system 30 may be provided for thepurposes of aligning the stencil 18 with the circuit board prior toprinting and to inspect the circuit board after printing. In oneembodiment, the imaging system 30 may be disposed between the stencil 18and the support assembly 28 upon which a circuit board is supported. Theimaging system 30 is coupled to an imaging gantry 32 to move the imagingsystem. The construction of the imaging gantry 32 used to move theimaging system 30 is well known in the art of solder paste printing. Thearrangement is such that the imaging system 30 may be located at anyposition below the stencil 18 and above the circuit board to capture animage of predefined areas of the circuit board or the stencil,respectively. In other embodiments, when positioning the imaging systemoutside the printing nest, the imaging system may be located above orbelow the stencil and the circuit board.

Referring now to FIGS. 2-4, the print head includes a frame 34 thatbecomes the gantry 22 when the print head 20 is installed so that theframe moves along a print direction, e.g., the y-axis direction.Specifically, the frame 34 is configured at its opposite ends to ridealong linear rails of the frame 12 of the stencil printer 10. Thisconstruction provides y-axis direction of movement of the print headgantry 22. The frame 34 supports a squeegee assembly generally indicatedat 36 having a pair of squeegee blades 38, 40 and a z-axis motionassembly configured to independently move the squeegee blades from araised position in which the squeegee blades are spaced from the stencilto a lowered position in which the squeegee blades engage the stencil18. As best illustrated in FIG. 3, the squeegee assembly 36 furtherincludes a front mount assembly 42 to mount the front squeegee blade 38and a rear mount assembly 44 to mount the rear squeegee blade 40. Thefront and rear squeegee blades 38, 40 are moved by motors 46, 48,respectively, mounted on the top of the frame 34. The arrangement issuch that the front and rear squeegee blades 38, 40 may be independentlylowered in the z-axis direction by the motors 46, 48, respectively, whenperforming print strokes with the squeegee blades. The squeegee assembly36 further includes sensors 50, 52 for detecting the home (i.e., raised)location of the front and rear squeegee blades 38, 40, respectively.

When each squeegee blade 38 and 40 is lowered onto the stencil 18, thesqueegee blade exerts a force on the stencil. This force is suitable forperforming a print stroke so that solder paste is rolled by the squeegeeblade and forced through apertures formed in the stencil. This solderpaste is deposited on the circuit board when separating the stencil 18from the circuit board by lowering the support assembly 28. The force ofthe squeegee blade on the stencil must be sufficient to force solderpaste (or any other viscous material) through the apertures formed inthe stencil, but not too great so as to damage the stencil.

Referring to FIGS. 5-7, for each squeegee blade, e.g., the rear squeegeeblade 40, a squeegee blade movement mechanism is configured to move thesqueegee blade from a raised position in which the squeegee blade isspaced from stencil 18 and a lowered position in which the squeegeeblade engages the stencil. With particular reference to FIG. 7 (alsoshown in FIG. 1), the squeegee blade movement mechanism comprises afirst movable member 54 coupled to the frame 34 of the print head 20coupled by the two linear bearings 57, and a second movable member 56coupled to the first movable member by the two linear bearings 57 andassociated springs. A lead screw 58, driven by one of the motors, e.g.,motor 48, is housed on the frame 34. The lead screw 58 threadablyengages a lead nut 60, which secured to the first movable member 54 tomove the first and second movable members between the raised and loweredpositions. An anti-rotation plate 61 is provided to prevent the rotationof the assembly when driving the lead screw 58. A compression spring 62is disposed around the lead screw 58 to provide a resistance forcebetween the first movable member 54 and the second movable member 56.

The arrangement is such that the motor 48 drives the first movablemember 54 in a downward direction (along linear bearings 57). Thisresults in the movement of the first and second movable members 54, 56being moved together. As soon as the squeegee blade 40 contacts thestencil 18, the compression spring 62 begins to compress resulting inthe movable members 54, 56 moving closer together thereby applying aprint force on the squeegee blade. The compression spring 62 provides aresistance to the movement of the first and second movable members 54,56 toward one another under print force application. As shown in FIG. 3,a squeegee blade holder 64 is configured to secure the squeegee blade 40to the second movable member 56.

As described above, sensors 50, 52 are provided for detecting thepositioning of the front and rear squeegee blades 38, 40 in their homepositions. In order to gauge the force of a squeegee blade, e.g., therear squeegee blade 40, against the stencil 18, a device is provided todetect a first reference point associated with a first force of thesqueegee blade against the stencil and a second reference pointassociated with a second force of the squeegee blade against the stencilwhen moving the squeegee blade to the lowered position. Specifically,the device comprises a flag 66 secured to the second movable member 56and a sensor 68 secured to the first movable member 54. Of course, thearrangement may be such that the flag 66 is secured to the first movablemember 54 and the sensor 68 is secured to the second movable member 56.The sensor 68 is configured to detect the flag 66 when moving thesqueegee blade to track at least two positions of distance between thefirst and second movable members 54, 56. Referring specifically to FIGS.5 and 6, the flag 66 is fabricated from bent metal and is configured tomove between a slot formed in the sensor 68. The flag 66 and the sensor68 may be secured to their respective first and second movable membersby fasteners, such as machine bolts.

Referring particularly to FIG. 7, the first and second reference pointsA, B are associated with the flag 66. The calibration or determinationof the particular forces associated with reference points A, B is asfollows. To calibrate the force of the engagement of a particularsqueegee blade 38 or 40 against the stencil 18, a first step of acalibration routine is to replace the squeegee blade with aforce/calibration gauge 70, which is provided to measure the force of asqueegee blade against the stencil by moving the gauge to the loweredposition. Also, the stencil is removed from the stencil printer. Thegauge 70 is lowered until it touches a rigid surface, such as a table ortooling top 72. The squeegee blade movement mechanism continues to lowerthe gauge 70 in known increments. At each known increment, the gauge 70records the force value, which is saved by the controller 14 within aprocessor configured to have squeegee force calibration routine, forexample. An example of recorded values is as follows:

Steps of Motion Force   0 0 1000 1 lb 2000 2 lbs 3000 3 lbs 50,000   50lbs

Next, the squeegee blade holder 64 is returned to the raised or startingposition (at 0 steps of motion). The gauge 70 is lowered until the “A”edge of the flag 66, which corresponds to reference point A in FIG. 7,first triggers the sensor 68. This force is recorded and stored withinthe processor of the controller 14. The gauge 70 is further lowereduntil the “B” edge of the flag 66, which corresponds to reference pointB in FIG. 7, triggers the sensor 68. This force is recorded and storedwithin the controller 14. The result is that the recorded forcescorrespond to the level of compression of the compression spring 62,which is linear and repeatable.

Next, the operator removes the gauge 70 and installs the squeegee blade40 onto the blade holder 64. The squeegee blade 40 is lowered until ittouches the stencil 18 to determine a zero reference point. The squeegeeblade is further lowered until the “A” and “B” edges of the flag 66 aredetected. These positions are compared to the reference points A and Bdetermined above to calculate the curve for the printing force of thespecific squeegee blade. In another embodiment, this two-step processmay be combined into a single process. Specifically, the system may beconfigured to start at a zero position and drive down the knownincrements through transition points A and B. The force values for allincrements and transition points are recorded for future reference.

In a certain embodiment, the flag 66 may embody a metal flag that ismounted to a fixed portion of the second movable member 56. In oneembodiment, the flag 66 contains two small notches, e.g., approximately1 mm in size, that are used to trigger the sensor 68. These notices maycorrespond to the “A” and “B” edges described above. The sensor 68 mayembody an optical interruption style sensor mounted to the floatingportion of the first movable member.

As discussed, the calibration of the print head may embody two separateoperations. Prior to the shipping of the stencil printer, a calibrationroutine is performed to characterize the squeegee head assembly withoutthe squeegee blades being installed. This calibration routine generatescalibration data that will reflect a linear relationship of forceasserted by the squeegee blades on the stencil. The second operation isa user-initiated squeegee blade force correction operation. In oneembodiment, this is a fully automatic correction operation, which may bedone after the squeegee blades are installed or changed by an operator.

With prior stencil printers, in order to obtain consistent andrepeatable print force data on both squeegee blades, an operator wouldneed to set the blades with different forces for the front blade and onthe rear blade. The reason for the difference in the applied forces wasdue to the compliance in the squeegee blades and the nonlinearconfigurations of the print heads. Once the blades are attached to thestencil printer, a squeegee blade spring compliance compensationcalibration is introduced to the squeegee head assembly. This introducedcompliance compensation calibration may require an additional correctionstep that a user can run (fully automatically) when the squeegee bladesare installed or changed. Once an operator runs a squeegee compliancecorrection routine, the data collected may be projected onto themanufacturing data to generate the corrected squeegee head z-axisdisplacement for achieving a desired force on the stencil.

In one embodiment, the z-axis print force may be between 0 lbs and 44lbs (0 kg and 20 kg), with a software setting resolution of 0.1 lb (0kg). With standard squeegee blades, e.g., and open-loop print head, theprint force repeatability may equal approximately +/−1 lb (0.45 kg). Anynumber of data points may be taken to obtain an accuratecharacterization of the force of the squeegee blades across and beyondthe 1 mm gap, for example, to achieve a desired resolution and force.

For example, the following approach may be employed to obtain thedesired values. This approach may be used if the mechanical system isdetermined to be linear. Below, assume that the following positions(Pos_(n)) and forces have been calibrated:

CalValue1—Pos₁ (0 mm=Force 0 lbs)

CalValue2—Pos₂ (1 mm=Force 4 lbs)

CalValue3—Pos₃ (2 mm=Force 10 lbs)

CalValue4—Pos₄ (3 mm=Force 20 lbs)

CalValue5—Pos₅ (4 mm=Force 44 lbs)

If the system is requested to apply a force of 7 lbs, the followinganalysis may be used to determine the interpolated position to move to:

First, the system must locate where the requested force falls for theabove-identified calibrated values. In this example, the force fallsbetween CalValue2 and CalValue3. Next, the system may employ thefollowing equation:InterpolatedPosition=Pos_(n)+((Force_((Pos n+1))−Force_((Pos n)))/RequestedForce)  (1)

-   -   where n=2

Thus, for the given example, the Interpolated Position is equal to 1.857mm, which is derived by performing the following calculation: 1mm+((10−4)/7)=1.857 mm

To factor for errors, the controller may determine the force expected atthe first transition point, i.e., the “A” location, and the forceexpected at the second transition point, i.e., the “B” location. Forexample, for location “A,” a minimum/maximum force may be between 1 lband 5 lbs. Thus, any force less than or more than this amount may beconsidered as an error wherein the operator is immediately notified andthe calibration test is aborted.

As described above, the print head may be manually calibrated using anexternal force gauge, such as gauge 70. Measurements of force (orpressure if using a pressure sensor) may be taken at specified squeegeehead displacements in the z-axis direction. From this information amapping is created between the displacement down and the force read offof the force meter. The calibration process may be broken into twophases. The first phase locates the first and second transition pointson the flag known as location “A” and “B.” These locations establish anaccurate force at two locations for establishing a linear compensationcurve for applying compliance compensation during a subsequent printprocess. The second phase of the calibration process consists of drivingthe squeegee head down at fixed distance intervals throughout therequired calibrated range for the print head and reading the forceapplied at each interval.

The first phase begins by testing the state of the sensor, which must bein an “off” position in order to start the process. If the sensor is notin the “off” position, an operator may be able to jog the squeegee bladedown too much in z-axis direction, or there may be a mechanical issuethat needs to be addressed. There should be very little or no down forceof the squeegee blade on the stencil before calibration starts. Next,the selected squeegee blade is moved under the operation of thecontroller at a jog velocity of 2 mm/sec (starting course movement)until the “A” location is detected. Once detected, the sensor willtransition from an “off” position to an “on” position.

Next, a downward z-axis movement of 0.25 mm will be issued to move intoa 1 mm gap area to search for the “B” location. The process of detectingthe “B” location applies the same approach as was used to detect the “A”location. The value for the “B” location point is also recorded. In theevent that a transition is not seen, the system will error out byreaching a software limit Once both “A” and “B” locations are found thesqueegee will reset to the zero force position so that calibration ofinterval forces across the flag can start.

The second phase begins at the end of the first phase by driving thegauge down at fixed intervals. The range of travel will consist of apreset number of points that will span beyond the “A” and “B” locations.At each interval the force displayed on the force meter will be enteredby the operator.

Thus, the first phase of the calibration process of the squeegee bladesmay be summarized as follows:

(a) initialize stencil printer;

(b) remove stencil from the stencil printer;

(c) remove the attached squeegee blades;

(d) locate the print head over the center of the substrate support;

(e) attach a force gauge to the bottom of the print head;

(f) turn on and set the force gauge;

(g) jog the substrate support up slowly until it comes in contact withthe force gauge;

(h) zero out the force gauge;

(i) enter a desired number of calibration intervals across entire rangeof travel;

(j) start the sequence;

(k) move the force gauge to detect the “A” location and the “B” locationon the flag, and return the force gauge to the zero force location;

(l) enter the force values as the squeegee head is driven down througheach interval;

(m) at the last interval, raise the squeegee head; and

(n) save the force and position data to the processor of the controller.

Whenever a situation arises in which the squeegee blades are changed,the operator will be given the opportunity to level the squeegee bladesto determine the z-axis height point at which the squeegee bladecontacts the stencil for that specific squeegee blade. As part of thisoperation, the system will automatically adjust for the squeegee bladecompliance. There may be some instances in which the stencil printer isconfigured in a manner where some tooling will not be compatible withthe compliance routine. In these situations, the stencil printer may beconfigured so that the compliance compensation routine may not occurautomatically—the operator will be prompted to perform the routine, ifdesired. In the instances in which the operator does not want to performthe compliance compensation routine, the stencil printer may beconfigured to perform the operation without the accuracy afforded byperforming the compliance compensation routine. Specifically, anoperator will be presented with an option on the display to select an“Auto Squeegee Force Correction.” When selected, the system will warnthe operator to make sure that both squeegee blades are installed andthat a stencil is loaded. The operator may be warned that the loadedsqueegee blades must be capable of handling a certain force, such as aforce of up to 25 lbs.

In either the automatic or the manual instance of the compliancecalibration process, the second phase of the calibration process of thesqueegee blades may be summarized as follows:

(a) the stencil is loaded into the stencil printer;

(b) the squeegee blades are both reinstalled;

(c) the operator moves the squeegee head over the center of the stencil;

(d) the substrate support (and printed circuit board) are moved to theprint position below stencil as they will be during this specific printprocess;

(e) the front (or rear) squeegee blade is selected;

(f) the squeegee head is moved to a home position;

(g) the front squeegee blade is lowered to print height, i.e., so thatthe front squeegee blade contacts the stencil;

(h) the front squeegee is driven down (lowered) into the stencil untilthe sensor detects the “A” position of the flag and automaticallyrecords the squeegee blade z-axis height for this transition point;

(i) the front squeegee will continue to be driven down into the stenciluntil the sensor detects the “B” location of the flag and automaticallyrecords the squeegee blade z-axis height for this transition point;

(j) the front blade data is saved into the database of the processor ofthe controller; and

(k) this process is repeated for the rear (or front) squeegee blade.

A squeegee force correction factor may be calculated and stored in adatabase each time a user-initiated squeegee force correction sequenceis executed. The correction factor also may be stored in a recipe filesince it will likely be consistent for the next time the same squeegeeblades are used for a specific product. A correction value is stored forthe front and rear squeegee blades. When a squeegee force is requested,the manufacturing calibration data may be scaled using the correctionfactor to determine the force displacement value.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. For example, the parameters described herein may bemodified to accommodate different printing process requirements.Accordingly, the foregoing description and drawings are by way ofexample only.

1. A method of calibrating a squeegee blade force against a stencil in a stencil printer, the method comprising: removing a stencil from a stencil printer; removing squeegee blades of a squeegee assembly of the stencil printer, the squeegee assembly comprising a squeegee blade movement mechanism configured to move the squeegee blades from a raised position in which the squeegee blades are spaced from the stencil and a lowered position in which the squeegee blades engage and apply a force on the stencil; attaching a gauge to a bottom of a print head; moving the gauge by the squeegee blade movement mechanism to detect a first position and a second position by engaging the gauge to a surface; recording a force parameter associated with the first position and a force parameter associated with the second position; determining a calibrated position for each of the first and second force values; and calculating and applying a desired print force based upon the calibrated positions of the first and second force values and actual positions of the first and second force values as detected by the gauge with at least one squeegee blade in place.
 2. The method of claim 1, further comprising: loading the stencil into the stencil printer; removing the gauge; re-installing the squeegee blades to the stencil printer; moving the at least one squeegee blade to contact the stencil; moving the at least one squeegee blade to the first position and to the second position; and recording a force parameter associated with the first position and a force parameter associated with the second position.
 3. A method of calibrating a squeegee blade force against a stencil in a stencil printer, the method comprising: moving a gauge in a downward direction against the stencil with a squeegee assembly including removable squeegee blades that are replaced with the gauge and a squeegee blade movement mechanism configured to move the squeegee blades from a raised position in which the squeegee blades are spaced from the stencil and a lowered position in which the squeegee blades engage and apply a force on the stencil; continuing moving the gauge in a downward direction to a first reference point; measuring the force of the gauge against the stencil for a first force value; continuing moving the gauge in a downward direction to a second reference point; measuring the force of the gauge against the stencil for a second force value; determining a calibrated position for each of the first and second force values; and calculating and applying a desired print force based upon the calibrated positions of the first and second force values and actual positions of the first and second force values as detected by the gauge with at least one squeegee blade in place.
 4. A method of calibrating a squeegee blade force against a stencil in a stencil printer, the method comprising: removing a stencil from a stencil printer; removing squeegee blades of a squeegee assembly of the stencil printer, the squeegee assembly comprising a squeegee blade movement mechanism configured to move the squeegee blades from a raised position in which the squeegee blades are spaced from the stencil and a lowered position in which the squeegee blades engage and apply a force on the stencil; attaching a gauge to a bottom of a print head; moving the gauge by the squeegee blade movement mechanism to detect a first position and a second position by engaging the gauge with a surface; measuring a force parameter associated with the first position and a force parameter associated with the second position; determining a calibrated position for each of the first and second force values; and calculating and applying a desired print force based upon the calibrated positions of the first and second force values and actual positions of the first and second force values as detected by the gauge with at least one squeegee blade in place. 